Polyethylene homopolymer which is prepared with a peroxide initiation in a high pressure polymerization process is a widely available item of commerce that is commonly referred to as high pressure/low density polyethylene (or “HPLD”). Such HPLD generally contains a significant amount of long chain branching (or “LCB”). The presence of the LCB reduces the density of the polyethylene. In addition, the presence of the LCB modifies the melt rheology of the polyethylene in a manner which is desirable for many fabrication processes—especially the production of blown film.
It is also known to produce “linear” ethylene homopolymers by the homopolymerization of ethylene with a coordination catalyst (such as a Ziegler Natta or “Z/N” catalyst). The resulting ethylene homopolymers are essentially free of LCB when prepared with a Z/N catalyst. These linear ethylene homopolymers have sharp melting points, which makes them suitable for some injection molding applications. In addition, plastic film which is prepared from linear ethylene homopolymer has high resistance to moisture transmission (or, alternatively stated, a low water vapor transmission rate or low “WVTR”). A review of plastic films having low WVTR is provided in U.S. Pat. No. 6,770,520 (McLeod et al.).
More recently, “linear” ethylene homopolymers have been produced with so-called single site catalysts (such as “metallocene” catalysts or “constrained geometry” catalysts). A review of homopolymers prepared from metallocene catalysts is given in U.S. Pat. No. 6,419,966 (Davis). Similarly, a review of linear ethylene homopolymers prepared with “constrained geometry” catalysts is provided in 2003/008,8021 A1 (Van Dun '03). In addition, the Van Dun disclosure provides a review of resin blends which contain an ethylene homopolymer blend component having a molecular weight distribution of greater than 2.5 (i.e. a blend component which is not prepared with a single site catalyst). Van Dun specifically teaches that “homopolymers derived from typical single site catalysts have the dual disadvantage of, (a) being unable to attain as high a density for a given molecular weight as comparable Ziegler products; and (b) exhibiting a narrow Mw/Mn across the whole molecular weight range.
We have now discovered certain blends of ethylene homopolymers which contain a homopolymer blend component having a polydispersity of less than 2.5, yet the blends have a surprisingly high density and thereby mitigate the disadvantages described by Van Dun '03.